A typical instantiation of a fibre optic burst mode ring network uses tunable lasers at the ingress transmit side, where the laser is tuned to a specific wavelength that is received by a specific location on the ring. Optical filters are used to drop different wavelengths at different locations on the ring. In this type of network ingress network traffic with a common destination is formed into bursts which are transmitted at the correct wavelength to get the bursts and traffic contained within to their destination.
In such networks there is an inherent unfairness as the location which is furthest upstream from the destination has un-contended access to that destination and can block closer sources. The unfairness arises because no two sources can add a burst at the same wavelength such that the bursts appear at the same position in the optic fibre ring. If this was to occur, both bursts would be corrupted and the information contained within lost. To prevent this from happening, the closer nodes must ensure they do not transmit at the same time as an upstream node. To clarify, the terms “furthest” and “closer” do not relate to geographical distance. They refer to the nodes position in the topology of a unidirectional ring. The furthest node is the node that attaches to the ring at the point most upstream from the destination node. The closest node is the once that is adjacent to the destination node. To provide an everyday analogy to the problem, cars entering a ring road from a slipway can be blocked by cars already on the ring road.
In the fibre optic spans between nodes in the ring, each wavelength cannot interfere with other wavelengths but if two transmitters put optical bursts onto a fibre optic span at the same wavelength at the same time they will interfere and be corrupted such that they cannot be received at the receiver. This is termed a ‘collision’ and occurrences of collisions should be avoided as these bursts will not be received and the lost data will have to be retransmitted. To avoid collisions each node must only add bursts onto wavelengths which are not occupied by bursts from other nodes and this leads to contention such that if two nodes wish to send bursts to one destination then they cannot transmit at the same time.
The network requires that only one node on the network can add the same wavelength at any one time to avoid wavelength collisions, which will corrupt the data. A wavelength collision avoidance scheme is disclosed in European Patent Number EP 1 759 558, assigned to Intune Networks Limited. This collision avoidance scheme allows for each node in the network to monitor the number of wavelengths currently used in the network. A control unit continually monitors at a node all the wavelength transmit data in the network and then can then decide which wavelengths are available for access on the network and select an add wavelength for transmit on the network to enable access. As each node operates asynchronously from the other nodes on the network this functionality means that each node can operate independently and monitor the available wavelength independently without the need for central control. The receiving node then uses a messaging channel wavelength to send a back-off or push-back signal to all the nodes that are trying to send data to it. This back-off signal is received by all nodes trying to send data, and each of these nodes then reduces the amount of time it is trying to send data to that receiving node by means of a fairness algorithm. For example, it may back-off by 50% which means that it only tries to access that wavelength 50% of the time that it was previously trying to do this. A problem with this fairness approach is that it is not an efficient way to use the bandwidth or wavelengths in the ring network. This mechanism is also more complex in that it relies on coordinated control, that is, the sources must receive push back messages from the receivers in order to achieve fairness.
Other mechanisms of wavelength access control for burst mode optical ring networks includes scheduling of time slotted access in synchronous optical burst mode ring networks. For “slotted rings” every node on the ring needs to be tightly synchronised. An interval of time is then divided into slots for each wavelength and a centralised scheduler allocates each node access to specific slots in the wavelength. The scheduling can be distributed, however every node needs to make exactly the same decision therefore the scheduling still requires global communications and the same scheduling operation at each node as is made in the centralised version. This method of wavelength access control is very complicated both in maintaining synchronisation across the ring and in scheduling access.
There is therefore a need to provide a ring wide wavelength access control in a fair manner without the need for synchronisation, distributed information or complex scheduling operations.